There's not a lot on the map to really talk about yet, but here are some things to think about with regard to integrating an element into an image.
General style: Obviously, with isometric mountains, you want other elements to also be isometric. That's not a hard and fast rule, but it's something to bear in mind. You wouldn't, for instance, combine these mountains with hachure lines in the hills, and you'll probably want tree symbols with trunks visible instead of an apparently top-down perspective. Likewise, you'll want colored line-art for other elements, not oil painting or watercolor styles or something like that.
Line quality: The coastline is not anti-aliased, but the mountains are. Examine the edges of the lines—see how there are intermediate colors that make the lines look a little smoother? That's anti-aliasing. You'll want to try to match those line qualities to one another. Gradients are another place where that kind of quality will show. You want to be sure that everything is drawn at roughly the same size so that filtering looks the same on all of them. If you rescale an element in either direction by too much, it will usually stick out.
Black levels: Be aware of the darkest parts of the map, and make sure that no elements go darker than that. Make sure that elements that are supposed to be equally dark have similar hues. If your darkest darks are reddish, but the line-work on the mountains tends toward the blue, then the mountains will look out of place, and it may be difficult to figure out why.
White levels: Like the blacks, whites are another thing that need to match. Pay especial attention to the brightest spots on your image, and make sure that those are the spots you actually want to be brightest. There are a lot of maps I've seen that can be improved by changing white features to taupe or gray.
Color harmony: I alluded to this in the previous two points, but you'll want to make sure your colors work well together. Choose a palette and try to get all of the elements to match that palette to some degree. Sometimes adjusting blend modes can make this really easy. Might as well cycle through them and see if one makes the mountains look really good. Sometimes it can be useful to make a copy of the layer, set the bottom copy to some blend that looks nice, then adjust the opacity of the top layer to let the blended colors show through a bit.
Interaction: Spread some shadows out from the mountains onto the land around them. Just a bit of a smudge at the base can work wonders sometimes. Also, the relationship between the elements can do a great deal sell the individual elements. Seeing a few mountains scattered across a landmass, it's hard to believe in them. When there are trees and rivers and villages around them, they may look much better.